(1) Field of the Invention
The present invention relates to an aircraft power plant, to an aircraft, and more particularly to a rotorcraft fitted with a power plant, and to a method of using a piston engine in a power plant having a conventional gearbox that is not designed for such a piston engine.
(2) Description of Related Art
In particular, the invention lies in the field of means for imparting rotary drive to a rotor fitted to the rotorcraft by using a piston engine, in particular a diesel engine. More precisely, the invention relates to a mechanism for attenuating the mechanical torque pulsations between such an engine and a conventional rotor gearbox that is not designed to be subjected to such torque pulsations.
Rotorcraft are generally fitted with at least one rotor that is driven by a power plant having a turbine engine and a power gearbox, said power gearbox being interposed between the turbine engine and the rotor. It should be observed that in the text below, the term “conventional” gearbox is used to designate a power gearbox that is adapted for coupling to a turbine engine.
However, e.g. for ecological reasons, it may be envisaged to make use not of a turbine engine, but rather of a piston engine. Although attractive, such a power plant appears a priori to require a power gearbox to be redesigned so as to be dedicated to a piston engine. Unlike a turbine engine, a piston engine generates torque pulsations that are liable to damage a conventional gearbox, since, by definition, such a gearbox is not designed to withstand such torque pulsations.
Furthermore, those torque pulsations are particularly large in the specific field of rotorcraft, because of the weight of such aircraft that needs to be compensated in order for them to take off and/or travel. More particularly, diesel engines may be used for driving rotors, but by their very architecture they generate very high levels of torque pulsations. The self-ignition of fuel in the combustion chamber gives rise to a sudden rise in pressure that induces instantaneous peaks of torque with a spectral response that presents high levels of harmonics up to high orders. Such torque pulsations, as conveyed by the mechanical transmission system between an engine and a rotor, can give rise to breakage due to fatigue in the members making up the transmission system, and in particular in the members of a conventional gearbox.
In order to damp such torque pulsations, a first solution consists in interposing a flywheel between a drive shaft and a driven shaft, the flywheel presenting a large amount of inertia in order to damp the torque pulsations. Such a solution presents the advantage of being simple to implement, but the drawback of increasing the size and the weight of the transmission system, which is something to be avoided in the field of aircraft such as rotorcraft.
A second solution consists in interposing a torsion mechanism between a drive shaft and a driven shaft. Conventionally, such a mechanism makes use of the twisting ability of one of the shafts, and more particularly of the drive shaft, by imparting a certain amount of flexibility thereto. The torsion shaft serves to absorb torque pulsations by having its resonant mode tuned well below the excitation that gives rise to torque pulsations. Such a solution presents the advantage of being simple to implement, but it makes it necessary to use a shaft of length and/or mass that become unacceptable for using such a shaft in the field of rotorcraft. Such a torsion shaft having twisting capacity suitable for absorbing torque pulsations is sometimes referred to as a “long” shaft by the person skilled in the art, but the term “torsion” shaft is nevertheless used below herein for convenience.
Document GB 358 789 envisages associating a torsion shaft with a clutch.